Remote sensing of visual, ultraviolet, and infrared sources currently employ electro-optical detection systems. As illustrated in FIG. 1, the detection systems are mounted on platforms supported by satellites 1, airplanes 3, ships 5 or land based vehicles 7, (FIGS. 1a, 1b, 1c and 1d, respectively). The detection systems include an electro-optical or optical sensor, typically a telescope, for collecting photons representative of a target and a detector for converting these collected photons into electrons and hence an electronic current. This enables information to be gathered about the target.
A prior electro-optical remote detection system for intelligence gathering applications which senses infrared radiation from a target is disclosed, for example, in a paper entitled "Defense Support Program: Support to a Changing World" by Kidd et al, given at the AIAA Space Programs and Technologies Conference, May 24-27, 1992 and the Infrared Handbook, published by ERIM for the U.S. Navy, 1989. Generally such systems seek a target signal in the presence of an overwhelming background signal. For example, these systems attempt to detect a missile from a down looking satellite against an earth background using a satellite sensor which covers a single band for scanning the exhaust signature of the missile. Such a single band detection system is described with reference to FIG. 2 which shows plots of relative signal power or intensity versus wavelength for a background signal 10 and a composite signal 11 of background and target. The background signal is almost always present in the band of interest. Typically, the background signal has undesirable "clutter" interference due, for example, to contiguous areas of high contrast. Severe detection problems often occur when, for example, the satellite scans a target having a sunlit earth behind it due to "glint" reflections of sunlight.
In FIG. 2, the background signal 10 closely approximates the composite signal 11 over the infrared (IR) spectrum between 2.0 and 5.0 micrometers, with the largest variation occurring between about 2.6 and 3.2 micrometers. Another significant difference exists between about 4.1 and 4.8 micrometers. The detection band 12 having a range of about 2.7 micrometers to 3.0 micrometers (see, e.g., page 2-76 of the Infrared Handbook) is typically chosen by the detection system for target recognition or extraction from the background. This corresponds to the water band exhaust signature which is prominent in a missile.
Conventional detection occurs when the intensity of the composite signal containing the missile exhaust within the band 12 exceeds the threshold 13. The limitation in this approach is that the detected intensity of the composite signal 11 must be perceptively larger than the background 10 within the detection band or false alarms will occur as a result of background variances. Texts in this field (see, e.g., Chapter 15 of Radar Handbook, M. Skolnik editor, 1970) have recognized the problem with false alarms in previous approaches. One attempt to solve the problem involves raising the threshold level 13 of the detector so that false detection of a target would occur infrequently. However, as shown, the composite signal 11 contains actual, but weak, target signals which are less than the threshold 13 over a portion of the detection band 12. Therefore, if the threshold level 13 is raised too high, many actual targets would remain undetected.
In another prior art approach, as used in the Defense Support Program (DSP), two discrete radiometers are used to cover two detection bands which are independently processed. However, such approaches can be defeated by a strong background signal that approximates or exceeds the target within its two relatively narrow detection bands. Hence it is also subject to a large number of false alarm indications when the detection threshold level is too close to the background clutter or to undesirably lower detection rates when the threshold is raised too high.